Wireless networks have experienced increased development, with mobile ad hoc wireless communications networks being one of the rapidly developing areas. Physically, a mobile ad hoc network includes a number of geographically-distributed, potentially mobile nodes wirelessly connected by one or more radio frequency channels.
Compared with other type of networks, such as cellular networks or satellite networks, the most distinctive feature of mobile ad hoc networks is the lack of any fixed infrastructure. The network is formed of primarily mobile nodes, and a network is created on the fly as the nodes transmit to or receive from other nodes. The network does not in general depend on a particular node, and dynamically adjusts as some nodes join or others leave the network.
In a hostile environment where a fixed communication infrastructure is unreliable or unavailable, such as in a natural disaster area struck by earthquake or hurricane, an ad hoc network can be quickly deployed and provide much needed communications. As wireless communications increasingly permeates everyday life, new applications for mobile ad hoc networks will continue to emerge and become an important part of the communications structure.
Mobile ad hoc networks pose serious challenges to the designers. Due to the lack of a fixed infrastructure, nodes typically need to self-organize and reconfigure as they move, join or leave the network. All nodes could potentially be functionally identical and there may not be any natural hierarchy or central controller in the network.
Many network-controlling functions are distributed among the nodes. Nodes are often powered by batteries and have limited communication and computation capabilities. The bandwidth of the system is usually limited. The distance between two nodes often exceeds the radio transmission range, and a transmission has to be relayed by other nodes before reaching its destination. Consequently, a network has a multihop topology, and this topology changes as the nodes move around.
The Mobile Ad-Hoc Networks (MANET) working group of the Internet Engineering Task Force (IETF) has been actively evaluating and standardizing routing, including multicasting, protocols. The IETF MANET Working Group has been evaluating ad hoc routing protocol proposals for consideration for standards track. Each protocol can be classified as either proactive, reactive, or some hybrid of the two. Over the last few years the group has narrowed its focus on a small suite of simple but flexible proactive and reactive protocols that are capable of addressing multiple deployment scenarios.
A limiting issue with both proactive and reactive routing approaches surfaces when scaling a network. This is usually addressed by the use of a hybrid approach with some level of combining proactive and reactive protocols. The proactive protocol usually addresses local area routing while the reactive protocol is used to discover routes to remote destinations outside the limited scope of the proactive routing protocol. The criteria for determining what is learned proactively verses reactively are in many cases specific to the deployment scenario. Proactive routing exchanges might be constrained by RF hops (hop radius) or possibly RF frequency (logical RF net), while reactive routing protocols cover those destinations falling outside the scope of the proactive protocol exchange.
As an example, FIG. 1 depicts a typical ad hoc wireless communications network 10 comprising wireless nodes arranged in a backbone net 20 interconnecting a plurality of stub nets 30, 40 and 50. The typical routing approach used in this type of network 10 is to exchange local routing knowledge within a stub net 30, 40 and 50 and exchange stub net address aggregates between wireless nodes 32, 42 and 52 defining gateway nodes in the backbone net 20.
Still referring to FIG. 1, a gateway node does not advertise routing to remote stub nets in its local stub net. Wireless nodes in the local stub net which need to get to remote destinations will default route that traffic to their gateway node. With this approach a packet routed from wireless node 38 to wireless node 58 would involve the use of a default route from wireless node 38 to gateway node 32, an aggregate route across the backbone net 20 between gateway nodes 32 and 52, and local routes between gateway node 52 and wireless node 58 in stub net 50.
However, this type of approach assumes that all wireless nodes maintain their membership in their assigned stub net or backbone net and are addressed appropriately for their net. This is a requirement so gateway nodes 32, 42 and 52 can advertise route aggregates to stub nets 30, 40 and 50 over the backbone net 20. This is a key scaling feature of this approach to keep backbone routing overhead manageable.
If for any reason a wireless node needs to move to a different stub net, then it typically needs to adopt that net's addressing scheme to maintain the efficiencies of route aggregation. The approach of changing a wireless node's address to fit a hosting net's address space introduces many new problems for network services, such as DNS, Routing and Security.
This impedes wireless node mobility, and therefore, the “ad hoc” nature of the network 10 because of the problems involved with a wireless node changing addresses dynamically to conform to a host net's addressing scheme. For example, if wireless node 56 in stub net 50 lost connectivity with other wireless nodes in stub net 50, but discovers it is in proximity of stub net 40, it should be able to join stub net 40. This type of scenario in an ad hoc tactical network is very real and should be addressed. In challenging or threatening theaters of operation it may be difficult to always maintain connectivity to your “home” network, thus requiring some automated means of net roaming without the complexities of changing wireless node addresses.
Flexibility is required in order to keep a user connected with their support groups. An ad hoc wireless communications network 10′ that improves somewhat on the above approach will now be discussed in reference to FIG. 2. Prime notation is used to indicate similar elements in alternative embodiments with respect to FIG. 1. In this example, wireless node 48′ is a member of stub net 50′ and wireless node 58′ is a member of stub net 30′, and are not addressed the same as their neighboring wireless nodes in each of the stub nets 30′, 40′ and 50′.
In this approach, a hybrid routing mechanism is provided in which each stub net 30′, 40′ and 50′ and backbone net 20′ exchanges only its local routing information proactively. If any two wireless nodes within the same stub net need to communicate, routes proactively exist to support this communications, similar to the above example. However, if there is a need to communicate with a wireless node outside the local stub net, such is the case between wireless node 38′ in stub net 30′ and wireless node 48′ in stub net 50′, a reactive routing protocol can be used.
As shown in FIG. 2, as in most reactive routing protocols, a route request message identifying the target wireless node 48′ is sent by wireless node 38′ and flooded through stub net 30′, through the gateway node 32′, and on to the backbone net 20′. In this case gateway node 52′ serving stub net 50′ will have a local route to target wireless node 48′ in stub net 50′, and can send a route reply message back to wireless node 38′ along the flooded discovery path building the route on the reverse path. In addition to this, gateway node 52′ can also send a gratuitous route reply onto wireless node 48′ to set up the reverse route from wireless node 48′ back to wireless node 38′ in the expectation that there will be a need for bi-directional communications between these wireless nodes.
This approach has an advantage over the approach in FIG. 1 since it does not require route aggregation, and therefore, does not rely upon a hard binding of a wireless node's address to their hosting stub net, making it more flexible in supporting wireless node mobility while keeping backbone routing overhead low. This approach is better served when a wireless node needs to migrate or roam to a new net.
A reliable assumption of any of these schemes is that traffic patterns in a hierarchical network tends to experience heavier local area traffic flows and lesser wide area traffic flows. Therefore, there is not always a need for every wireless node to proactively learn how to route packets to every other wireless node in a larger network. As well, it is assumed that each wireless node will have a preferred stub net that it is most likely to gravitate to when within range. This is also most likely where most of its communications will occur due to the group's common objectives.
The hybrid approach described in FIG. 2 relies on the flooding of discovery messages to establish routes to remote wireless nodes. With this mechanism there is some risk of network congestion if many users need access to remote services simultaneously causing a flurry of reactive route discoveries. A scenario that could cause this need for simultaneous discovery might be a sudden link change close to a commonly accessed server requiring all wireless nodes to re-establish connectivity. This sudden spike in flooding of discovery messages can negatively impact scalability and performance of the mobile ad hoc network, including a decrease in bandwidth due to an increase in network overhead.